# How to substitute $x$ in a expression $f(x)$ but not calculate the value of the $f(x)$ at the point $x$?

I want to subtitute $x = 1$ into the expression $$f(x)=x^2 + 3 (x^2 - m^2) + \sqrt{x^2 + 1}$$ but I don't calculate the value $f(1)$, that is mean, I want $$1^2 + 3(1^2-m^2)+\sqrt{1^2 + 1}.$$ If I tried

f[x_] := x^2 + 3 (x^2 - m^2) + Sqrt[x^2 + 1];
f[1]

1 + Sqrt[2] + 3 (1 - m^2)

How do I tell Mathematica to do that?

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Dirty trick : f["1"]. –  b.gatessucks Feb 26 '13 at 8:21
Thank you very much. –  minthao_2011 Feb 26 '13 at 8:22